Aircraft having aft-mounted main wings often have engines positioned near the trailing edge of the main wing to counterbalance the weight of the fuselage extending forward of the main wing. In addition, such aircraft generally have main landing gears (“main gears”) extending downward from the main wing forward of the engines to support the aircraft when on the ground. The relative positioning of the engines and the main gears in this configuration can raise a number of design problems.
One problem is the potential for foreign object damage (FOD) caused by debris kicked up by the main gear and ingested through an engine air inlet. One approach to overcome this problem has been to extend the engine air inlet forward to position the inlet aperture in front of the main gear. Another approach has been to route the engine air inlet through the wing to position the inlet aperture above the wing and shield it from the main gear. Yet another approach has been to install a screen or similar device over the inlet aperture to prevent FOD.
All of these approaches for preventing FOD have disadvantages related to aircraft weight, complexity, and drag. For example, extending the air inlet forward of the main gear increases airframe weight. Similarly, routing the air inlet through the wing not only increases airframe weight but also increases the structural complexity of the airframe. Further, providing a screen or the like over the inlet aperture has the disadvantage of increasing aerodynamic drag and reducing inlet efficiency.
Mounting the main gears to the wing can also raise a number of design problems. One problem is that the wing structure must be tailored to provide a wheel well and carry the main gear loads. This typically requires adding significant structural reinforcement around the wheel well and providing a substantial truss structure for mounting the main gear, both of which can add considerable weight to an airframe.
Another problem related to wing-mounted main gears is preventing a fuel tank puncture in the event of a main gear collapse. On most transport aircraft, the fuel tanks in the wings (“wing tanks”) carry most of the fuel for the aircraft. If a main gear collapses beneath a wing tank, the main gear could puncture the wing tank. This problem is typically solved by not carrying fuel over the main gear, thus providing a “dry bay” in this region of the wing.
The dry bay solution has a number of drawbacks. One obvious drawback is the resulting reduction in fuel capacity. Another drawback is the unfavorable effect the dry bay has on aircraft balancing characteristics. Fuel in the wing tank can often be used to favorably balance the aircraft about its center of gravity (CG). The reduction of wing tank capacity caused by the dry bay, however, may require that other methods be used to balance the aircraft about the CG. Often, these other methods involve aerodynamically trimming the aircraft, which has the unfavorable effect of increasing the aerodynamic drag of the aircraft.